Sometimes when I get stuck with this question myself, I break it down into more manageable components. To be considered a Christian, you have to believe in God, believe that Jesus died for the forgiveness of all sins and to secure salvation, believe in the gospel, and live a Christ-like life. The more you get into the specifics of it, that's when you start branching into different sects of Christianity: Catholicism, Protestantism, Evangelicalism, etc. If you live your life in accordance with these beliefs, then you're considered Christian. The reasons adding witchcraft (craft, not Wicca) to the mix becomes problematic is because there are some who deemed it a practice which allows the user to have self power and self reliance, things that Abrahamic God believers are only supposed to get from...well...God. Personally, I think if you're going to go by the book, so long as you practice in a way that has your power and control ultimately in the hands of God, then that is what would constitute a true Christian witch - a God believing/loving/worshipping person who practices craftwork and lives their life according to Christ's example. When combining Wicca and Christianity, you're trying to meld together two very different systems of belief, and reconcile them in such a way that they make sense and work well together...but personally - and again, this is just my opinion - I don't think it works out too well. Sure, you can take elements of Wiccan practice and incorporate Christian themes, but to some, this would be considered impossible, trying to balance and live with one foot in two different worlds.
If it's something that truly means a lot to you to discern, take the time to talk to fellow Christians, Christian witches, Wiccans, regular witches, and - if you can find some solid ones - Christian Wiccans. See how they live their lives, check out their belief systems, and see what does/doesn't fit into what you're trying to accomplish here. Spend time taking it up with God personally through prayer. Ultimately, you know you best, and if being authentic and genuine with your practice is what you're striving for here, then that's all that really matters. :)